Nervous System Divisions and Functions

• The nervous system acts as the body's control center, fulfilling three primary roles.
• Gathering sensory input
• Integration through processing and decision-making
• Generating motor output to initiate responses

Divisions of the Nervous System

• The central nervous system (CNS) consists of the brain and spinal cord.
• Integrative control centers oversee the system.
• The peripheral nervous system (PNS) exists outside the CNS and includes cranial and spinal nerves.
• It serves as a communication pathway to and from the CNS, and has two divisions.
• Sensory Division transmits impulses from muscles, skin (somatic) and organs (visceral) to the CNS.
• Motor Division transmits impulses from the CNS to muscles and glands.
• The somatic nervous system controls skeletal muscle (voluntary actions).
• The autonomic nervous system (ANS) regulates involuntary actions via smooth and cardiac muscles.
• This system also controls glands.
• Has two divisions: sympathetic and parasympathetic
• Sympathetic division mobilizes the body for "fight or flight" responses.
• Parasympathetic division promotes energy conservation through "rest and digest" functions.

Cells of the Nervous System

• Neurons are excitable cells that transmit nerve impulses.
• Neuroglia are supporting cells within the nervous system.
• Neuroglia within the CNS include:
• Astrocytes attach neurons to capillaries and regulate the chemical environment.
• Microglia monitor neuron health and act as phagocytes.
• Ependymal cells line the CNS's fluid-filled spaces and circulate fluids using cilia.
• Oligodendrocytes create myelin sheaths around CNS neuron axons.
• Neuroglia of the PNS include:
• Satellite cells insulate neuron cell bodies and provide electrical insulation.
• Schwann cells (neurolemmocytes) form myelin sheaths around PNS neuron axons.

Neuron Structure and Function

• Neurons (nerve cells) are functional units of nervous tissue - conduct impulses.
• Neuron characteristics:
• Longevity - lasts a lifetime
• Amitotic - cannot divide once mature
• High metabolism - consumes much oxygen and nutrients
• The soma (cell body) contains the cytoplasm, nucleus, and organelles.
• Dendrites are short, numerous processes that carry input toward the soma.
• The axon relays impulses away from the soma.
• Terminal branches are divisions at the end of the axon.
• Axon terminals are bulbous structures at the axon's ends and contain neurotransmitters.

Neural Tissue Organization

• In the CNS, a nucleus is a cluster of neuron cell bodies.
• Ganglia are clusters of neuron cell bodies in the PNS.
• Tracts are axon clusters in the CNS, while nerves consist of axon clusters in the PNS.

Myelin Sheaths

• Myelin sheaths consist of fatty, white wrappings around nerve fibers.
• They insulate the fiber and speed up impulse transmission.
• Schwann cells (neurolemmocytes) in the PNS wrap plasma membranes many times around an axon to form myelin sheaths.
• The neurilemma is the bulging portion of a Schwann cell, what remains of the cell.
• Nodes of Ranvier are gaps between adjacent Schwann cells along the axon.

Neuron Classification

• Structural classification of neurons
• Multipolar neurons are the most common type, featuring multiple dendrites and one axon.
• Bipolar neurons are rare, with one dendrite and one axon.
• Unipolar neurons possess a single process (an axon) and are involved in receptive and secretory functions.
• Functional classification of neurons
• Sensory (afferent) neurons transmit impulses to the CNS.
• Motor (efferent) neurons transmit impulses from the CNS to effectors (muscles or glands).
• Interneurons are located between sensory and motor neurons within the CNS.

Electrophysiology Principles

• Voltage (potential) is the potential energy generated by separated charges (Na+ and K+).
• Current is the flow of electrical charges from one point to another (Na+ and K+ movement across the cell membrane).
• Resistance hinders charge flow (cell membrane impedes Na+ and K+ flow).
• Ohm's Law defines the relationship between these factors: Current = Voltage/Resistance.

Membrane Ion Channels

• Ion channels are membrane proteins used to diffuse ions across the cell membrane - creating electrical currents.
• Nongated (leakage) channels are open all the time.
• Chemically gated channels open upon binding specific chemicals.
• Voltage-gated channels open in response to changes in the membrane potential.

Resting Membrane Potential

• Resting membrane potential is defined as the voltage measurement across a neuron's cell membrane.
• The membrane is polarized, with the cytoplasm exhibiting a negative charge, typically -70mV, relative to the outside.
• More K+ diffuses out of the cell than Na+ diffuses, making the inside of the cell negative.
• Na+/K+ pump maintains constant potential.

Changes in Membrane Potential

• Depolarization is a potential reduction, decreasing the polarity of the cell causing the inside to become less negative.
• Hyperpolarization increases membrane potential – increasing the cell's polarity makes the inside more negative.

Graded Potentials

• Localized changes in membrane potential define graded potentials.
• These potentials travel and dissipate over short distances.
• Typically, they occur in dendrites and somas.
• Graded potentials respond to a stimulus
• Graded potentials can be depolarizations or hyperpolarizations
• Graded potentials have the ability to initiate an action potential.

Action Potentials

• Defined as large changes in membrane potential.
• Action potentials travel long distances while maintaining strength.
• Typically seen in axons of neurons (nerve impulses) and in muscle fibers.

Action Potentials Phases

• Resting state: Membrane is polarized at -70mV. All gated ion channels are closed.
• Depolarization: Stimulus opens Na+ channels, Na+ flows into the cell, and the membrane reaches +30mV.
• Repolarization: Na+ channels close, K+ channels open, K+ flows out, and membrane returns to -70mV.
• Hyperpolarization: K+ channels remain open, and the membrane potential dips below -70mV.
• Ion Redistribution: Na+/K+ pump restores ions to resting concentrations.

Propagation

• Propagation is how cells maintain action potential down the axon.
• Each spot depolarizes, repolarizes, then maintains state.
• This causes a cascade down the axon.
• Depolarization opens voltage gated downstream channels.
• The voltage-gated channels reside along the Nodes of Ranvier if it is a myelinated axon.
• Increased diameter of the axon - increases rate of propagation.

All-or-None Phenomenon and Threshold

• The all-or-none principle dictates that an action potential either occurs completely or not at all.
• The threshold is the minimum voltage required to trigger an action potential, approximately -55mV.

Refractory Periods

• These dictate plasma membrane generation time.
• No generation or difficult to generate during these periods.
• During the Absolute Refractory Period, Na+ and K+ channels are open, which makes it impossible to generate another AP during this period.
• During the Relative Refractory Period, K+ channels remain open, an AP is difficult to generate during because of a required higher threshold.

Synapses

• Synapses are the junction between two neurons, or a neuron and the structure it activates (muscle or gland).
• The presynaptic neuron is the sender of information, releasing neurotransmitters from its axon terminal.
• The postsynaptic neuron is the receiver of information, acquiring neurotransmitters, typically on its dendrites.
• At the axon terminal, an action potential triggers the opening of Ca2+ channels.
• Calcium influx causes neurotransmitter release from synaptic vesicles.
• Neurotransmitters travel and bind to postsynaptic receptors.
• Neurotransmitter binding opens postsynaptic ion channels, initiating a graded potential.

Removal of Neurotransmitters

• Neurotransmitters are removed to stop synaptic activity.
• Neurotransmitters flow away from the cleft via Diffusion.
• Recycling into the presynaptic cell or local astrocytes is done via Reuptake.
• Enzyme Degradation has the enzyme breakdown the neurotransmitter into a nonfunctional form.

Postsynaptic Potentials

• The postsynaptic neuron sees different types of graded potentials
• Excitatory Postsynaptic Potential (EPSP) occurs:
• When a neurotransmitter moves cell membrane voltage towards threshold, the cell encouraged towards action potential)
• A net, influx of Na+ ions causes this.
• Inhibitory Postsynaptic Potential (IPSP) causes:
• Movement away from firing threshold (inhibitory) due to neurotransmitter (either Cl- influx, or efflux of K+)
• Prevents AP.
• Summation is the additive influence of many EPSPs, as one EPSP cannot trigger an action potential.

Neurotransmitters

• Structural classification of neurotransmitters:
• Acetylcholine (ACh) - Neurotransmitter at neuromuscular junctions and in the autonomic nervous system.
• Biogenic Amines - regulate emotion/biological clock - Includes dopamine, serotonin, norepinephrine, and histamine.
• Other - includes Proteins, Amino Acids, ATP, and dissolved gases.
• Functional classification:
• Excitatory - Depolarization of a neuron (EPSP).
• Inhibitory - Hyperpolarization (IPSP).
• Some can be both because neurotransmitters rely on whichever receptor it attached to.